Wearable heart rate monitoring device for fitness is a fast growing market. The devices using photoplethysmograph (PPG) signals for heart rate monitoring have received significant attention in recent years. A PPG signal is an optically obtained plethysmogram, a volumetric measurement of an organ. An LED (light emitting diode) illuminates a person's skin, and a photodiode measures light absorption or reflection. The photodiode produces a PPG signal indicative of the measured light absorption or reflection. Changes in this PPG signal may be used to detect the heart rate.
The devices produce real-time estimate of heart rate from the PPG signal recorded from the wearer. A heart rate monitoring device includes a heart rate monitoring (HRM) circuit which is responsible for estimating the heart rate. However, when the person wearing the heart rate monitoring device is in motion, a motion signal strongly interferes or drowns the heart rate signal. Thus, it is important to remove the motion signal before heart rate determination.
One conventional method to separate the heart rate signal and the motion signal is to use singular spectral analysis (SSA). The PPG signal is mapped into a matrix and a singular value decomposition of the matrix is performed. The singular values and the singular vectors corresponding to the motion signal and the heart rate signal are identified and separated. The matrix corresponding to the heart rate signal is remapped to the heart rate signal for further processing. However, this method is highly complex as it requires performing matrix operations. Also, this method requires intensive computation to identify the heart rate signal as part of signal decomposition and reconstruction.
Another important requirement in a heart rate monitoring device is that, it should be of low complexity with small form factor and low power consumption as these are battery operated devices.